The air return of most air conditioning systems is provided with an air filter to remove airborne dust and other airborne contaminants that could clog and/or cover the coils of the evaporator or heating element of the air conditioning system. These air filters become clogged with filtered contaminants and must be periodically cleaned or replaced in order to avoid a decrease in air conditioning efficiency. It would be a benefit, therefore, to have a monitoring device that could be attached to the air return intake of an air conditioning system that could detect and alert the person charged with replacing or cleaning the air filter when such maintenance was required.